The invention relates to a method for removing aluminum and/or fluorine and/or iron from impure phosphoric acids and involves processes for producing novel, monoclinic, aluminum fluorophosphates (or fluoroaluminum phosphates) of composition AlFHPO.sub.4.2H.sub.2 O (or FAlHPO.sub.4.2H.sub.2 O) which contain iron and are essentially free of calcium, are hygroscopic, are usually layered, and can be readily decomposed (in strong sulfuric acid or by heating) to recover hydrogen fluoride. When decomposed by heating, the aluminum fluorophosphate can produce a solid aluminum phosphate which can be used as a fertilizer, or converted to sodium phosphates, etc.
The present invention also involves a means of removing aluminum and fluorine from impure phosphoric acids and means of converting high alumina content phosphate matrix into a relatively low aluminum content phosphoric acid without the usual beneficiation by flotation. This process permits conversion to phosphoric acid (and aluminum phosphate) of a much greater proportion of the phosphate values in the matrix. For example, in the usual beneficiation of phosphate matrix, by the dihydrate or hemihydrate routes, only about fifty percent of the phosphate values in the matrix are recovered in the beneficiated product. In contrast, about eighty percent of the phosphate values in the matrix can be recovered by the present process.
Phosphate reserves are sedimentary deposits formed by reprecipitation of dissolved phosphate from prehistoric seas. For example, a typical North Florida phosphate ore consists primarily of fluorapatite (a phosphate-containing mineral), quartz sand, and clay minerals. This ore body is called the phosphate matrix.
In current mining practice, the matrix is excavated by draglines, slurried with water at high pressure (e.g. about 200 pounds per square inch) and pumped through miles of pipeline to the beneficiation plants where sand and clays are removed from the fluorapatite by flotation processes, producing the so-called beneficiated phosphate rock.
Current commercial processes call for the usage of either beneficiated or high quality phosphate rock and sulfuric acid as raw materials to produce either hemihydrate or dihydrate phosphoric acid. In most cases where beneficiation operations are required, losses of about 40% P.sub.2 O.sub.5 values in the matrix occur in the form of slimes and tailings. The slimes are discharged to storage ponds as a dilute slurry containing about 5% of fine particulate minerals, which take years to settle. For every acre-ft of matrix mined, about 1.5 acre-ft of slime is produced as a result of beneficiation. Accordingly, rock beneficiation creates an environmental concern in addition to the large loss of P.sub.2 O.sub.5 values.
In U.S. Pat. No. 3,792,151 to Case, phosphoric acid is produced from low BPL (bone phosphate of lime or tricalcium phosphate) phosphate rock having about 1.5% fluorine by a process comprising reacting the phosphate rock with an equilibrated phosphoric acid (as defined in the Case Patent) having a P.sub.2 O.sub.5 concentration between about 20 to 50% in an attack stage at a temperature below about 180.degree. F., the equilibrated acid being essentially saturated with respect to the fluorine component of said rock at the temperature of the attack stage; the temperature and the time of reaction serving to dissolve at least about 90 percent of the tricalcium phosphate in the rock to produce a monocalcium phosphate-phosphoric acid-water solution up to about 90 percent saturated with monocalcium phosphate and containing insoluble material and a soluble fluorine content of from about 1 to 3 percent, the weight ratio of P.sub.2 O.sub.5 in the acid to P.sub.2 O.sub.5 in the rock feed being sufficient to dissolve tricalcium phosphate values in the rock and provide the desired solution and at least about 7:1, separating the insoluble material from the solution to produce a solution of monocalcium phosphate-phosphoric acid-water (said solution having a fluorine content of from 1 to 3 percent), reacting sulfuric acid with said solution to produce phosphoric acid and precipitate calcium sulfate, the sulfuric acid being added in an amount essentially stoichiometric with respect to the monocalcium phosphate in the solution, separating the calcium sulfate from the phosphoric acid solution, removing a portion of the phosphoric acid as product, and recycling the remaining phosphoric acid solution to the attack stage to provide said equilibrated acid and removing a portion of the phosphoric acid as product.
There is no disclosure in the Case patent of a process for removing aluminum from the product acid by forming an aluminum fluorophosphate. Furthermore, the phosphoric acid produced by the process of the present invention is not an equilibrated phosphoric acid (as defined in the Case Patent) because of the removal of the aluminum and fluorine in the precipitation of the fluoroaluminum phosphate. In one embodiment of U.S. Pat. No. 282,466 of Ore, now U.S. Pat. No. 4,377,560, this low aluminum and low fluorine content phosphoric acid product is used to dissolve the tricalcium phosphate in the matrix; thereby controlling the fluoride content of the crystallization (of calcium sulfate) step such that the fluoroaluminum phosphate does not form until after the gypsum separation. The Oree application also involves controlling the fluoride content by other means, such as volatilization and addition of sodium or potassium compounds.
In the manufacture of synthetic cryolite, an aluminum fluorophosphate AlF.sub.2 H.sub.2 PO.sub.4 is reported in the U.S. Pat. No. 3,671,189 to Betts. This composition is different from that produced in the present process, in that it is relatively higher in HF than in the AlFHPO.sub.4 of the present invention. Also, the production of the Betts compound would not lower the aluminum content of phosphoric acid to as great an extent as does the production of AlFHPO.sub.4.2H.sub.2 O as disclosed hereinafter. The process steps involved in the manufacture of synthetic cryolite are quite different from the process for manufacture of the novel fluoroaluminum phosphate of the present invention.
M. S. White, New Zealand Journal of Science, Volume 19, pages 421-431 (1976) discloses chemical analyses of chemical compounds in superphosphates made from phosphates of Christmas and Nauru Islands and Queensland. One reported compound was termed "RPF" and assigned the formula: (Fe,Al)HPO.sub.4 F.2H.sub.2 O. X-ray powder diffraction patterns were reported (at page 425) for "RPF" and for "synthesized material, Al analogue only." A chemical analyses of this latter material is given at page 427 of the paper. In the White paper, at Table 13, page 429, thermal degradation data is shown and the compound (Fe,Al)HPO.sub.4 F.2 H.sub.2 O is reported to have the analysis:
1.82% Fe,10.4% Al,0.87% Ca,14.9% P and 10.5% F.
As used herein, the term "Falphite" (sometimes spelled "Falfite") means a precipitate from impure phosphoric acid (as opposed to pure materials) having the composition AlFHPO.sub.4.2H.sub.2 O and also containing iron (due to iron in the phosphate rock or matrix, from which the phosphoric acid was made) and containing less than 0.2 wt.% Ca.
In the present invention, the compound termed "Falphite" typically contains less than about 0.1% Ca, and usually no detectable Ca. Furthermore, the Fe.sub.2 O.sub.3 analysis of such Falphite is usually less than about 3% (e.g. 2.35%).
Fluoroaluminum phosphate of composition FAl(HPO.sub.4).2H.sub.2 O is reported in the July 1980 Russian Journal of Inorganic Chemistry 25(7) 1980; however, this compound is reported as being formed by a process involving adding aluminum sulphate solution to a mixture of phosphoric acid and ammonium fluoride. The reagents used were "pure" or "highly pure" grades. No work is reported in the Russian Journal article of a process whereby FAlHPO.sub.4.2H.sub.2 O or F(Fe,Al)HPO.sub.4.2H.sub.2 O is prepared from impure phosphoric acid (e.g., green or black acid or from a high alumina content phosphoric acid produced from unbeneficiated matrix)
J. W. Akitt, N. N. Greenwood, and G. D. Lester, "Nuclear Magnetic Resonance and Raman Studies of the Aluminum Complexes formed in Aqueous Solutions of Aluminum Salts Containing Phosphoric Acid and Fluoride Ions," J. Chemical Society (A), 1971, mention the existence in a liquid phase of the complex AlF.sub.2 H.sub.2 PO.sub.4.
Herein percentages are by weight unless otherwise specified.